The present invention relates to a fiberoptic display for displaying an image in a region having a strong magnetic field. A coherent fiberoptic bundle transfers the image across a gradient between regions having different magnetic flux densities. The present invention is particularly well suited for transferring a video image from a cathode ray tube (CRT) to a fiberoptic display scope positioned in the imaging zone of a magnetic resonance imaging (MRI) system.
The image transfer system for the fiberoptic display of the present invention applies to transferring any optical image across the gradient between a region of high magnetic flux density and a region of low magnetic flux density. The invention will be illustrated through application to the problem of projecting a video image into the imaging zone of an MRI system.
Magnetic resonance imaging is a well known process that creates a three dimensional image of the distribution of magnetic dipoles in a body. The resulting images can be used to make medical diagnosis. The process involves positioning the patient in an imaging zone of a MRI apparatus for approximately 40 minutes. The MRI apparatus generates a strong magnetic field around the patient to create a region having a high magnetic flux density. This magnetic field is systematically changed so as to change the orientation of the dipoles in the patient. The three dimensional image is constructed from measurements made of the changes the orientations of the dipoles.
The art recognizes that the resolution obtained using magnetic resonance imaging is degraded or destroyed if the patient moves during the imaging process. Motion can result from the normal functioning of the patient's respiratory and cardiovascular system. Furthermore, the process of making a magnetic resonance image causes anxiety. The patient must be placed in the confined space of the imaging zone of an MRI apparatus which can induce feelings of claustrophobia. The MRI apparatus itself is shielded from its surroundings to attenuate the magnetic field. The shielding further isolates the patient in the imaging zone. The MRI apparatus makes a noise during the imaging process that is similar to a power transformer. Some say that such a strong magnetic field is perceptible and produces a strange sensation. The extended confinement, unfamiliar surroundings and sensations can create anxiety--particularly for children. A natural response to anxiety is nervous behavior such as twitching or fidgeting. This motion causes a physical displacement of the patient that can blur and destroy the magnetic resonance image. The imagining process must then be lengthened or repeated which can create even more anxiety.
It is recognized in the art that patient motion can blur or destroy a magnetic resonance image. Electronic systems have been used to compensate for motion by modifying the way that the magnetic resonance image is formed. These electronic systems often seek to eliminate the effects of respiratory or cardiovascular functions by exploiting their periodic motion. The effects of motion caused by the physical displacement of the patient, however, are far more difficult to eliminate with electronic systems because this type of motion is not predictable. The electronics needed to compensate for the physical displacement of a patient needs to be highly complex and therefore also quite expensive.
It is known in the art that anxiety can be reduced by distracting the patient with a television. A person who is distracted by a familiar show is less frightened by confined surroundings and strange sensations.
While these general principles are known, their application to magnetic resonance imaging is not simple. For example, the CRT used to create a video image in a television would not function in the strong magnetic field created in the imaging zone of the MIR system and may be disturbed outside the shield by any magnetic flux that leaks out. A CRT forms an image by scanning an electron beam across the phosphor screen that forms the back side of the display screen. The electron beam is scanned by deflecting it with magnets. The beam can be disturbed or totally overwhelmed or by the magnetic field created during magnetic resonance imaging. The electron beam of a CRT would not be able to scan the phosphor screen in a region of high magnetic flux density. A CRT therefore could not produce an image in the imaging zone during magnetic resonance imaging.
The image formed by a solid state display of liquid crystal diodes (LCD's) also could be disturbed by the high magnetic flux density. Any electric current induces a magnetic field that will interact with any other magnetic field nearby. The electric signals used to control a solid state display could therefore be destroyed by interaction with the magnetic field in the imaging zone of an MRI system. The systematic changes in the magnetic LCD field would make a display even less practical.
The electrical signals needed to generate any video signal would likely disturb the magnetic resonance image. The induced field from a video display could be stronger than the dipoles of the atoms and molecules that the MRI system is designed to measure. The resulting disruptions would be difficult to eliminate.
The only way thus far proposed to communicate with a patient undergoing magnetic resonance imaging is to supply sound to the imaging zone using an air hose system similar to that used to supply music to the passengers on many aircraft. The air hose system avoids the problem of transferring an electromagnetic signal into the imaging zone of an MRI system by transferring the music through air rather than through a wire. Sound is not an electromagnetic signal and is therefore not effected by high magnetic flux density.
The need to eliminate the adverse effects on the magnetic resonance image caused by the motion of the patient have been addressed by electronic systems that reduce the effects of image blurring. No system other than sound delivered by air hoses is known that could communicate with a patient during magnetic resonance imaging. One solution that has been attempted is to equip the patient with glasses made of a magnetically inert material to reflect a scene from outside the imaging zone into the field of view of the patent. Clinical tests have shown that these glasses are effective in easing anxiety in patients prone to claustrophobia. The glasses, however, are not known to project a moving image to a patient and therefore do not provide an active distraction to relieve more general feelings of anxiety.
A need exists in the art to reduce the tendency of a patient to move while undergoing magnetic resonance imaging by displaying a video image in the measuring area of an MRI system to distract the patient and thus increase the efficiency of the magnetic resonance image by reducing the number of scans that must be redone.